The present invention relates generally to radiometers and more particularly to a Fast Fourier Transform (FFT) based digital radiometer.
A Radiometer is used on board a spacecraft to measure weather parameters from space. One function of a radiometer is to measure the temperature of the atmosphere as a function of altitude. At a specific portion of the frequency spectrum, i.e. 60 GHz, oxygen becomes very absorptive, making it ideal as a frequency to measure the temperature of the atmosphere. The radiometer measures the intensity of upwelling radiation in narrow sections, or channels, of the frequency spectrum around 60 GHz; ground based algorithms use this data to and produce a measure of the atmospheric temperature as a function of altitude.
Typically, radiometers are analog devices that have a separate filter for each narrow section of frequency. However, the filters have limitations on how close they can be spaced from each other. Due to packaging constraints on board a spacecraft, the spacing between filters is usually wider than it should be and filter spacings are typically non-uniform.
The analog approach, which has wide filter channels, tends to average out the spectral detail. The result is a lower resolution temperature profile, and ultimately a less accurate temperature measurement. Additionally, the vertical resolution of the air temperature profile is limited by achievable channel bandwidths.
The analog radiometer consists of a set of analog filters that do not provide continuous spectral coverage. The analog radiometer typically requires multiple channels and requires a separate filter for each channel. Subsequently, each filter has a separate design and each filter must be tuned. Further, Doppler correction for analog radiometers requires local oscillator tuning which causes changes in filter gain.
It is an object of the present invention to provide a Fast Fourier Transform based digital radiometer that can completely characterize an oxygen absorption line. It is another object of the present invention to provide greater spectral detail than prior art analog approaches. It is still another object of the present invention to eliminate the need for tuning individual filters.
It is a further object of the present invention to provide Doppler correction to account for the velocity of a spacecraft and the direction of the area being measured. It is yet a further object of the present invention to compensate for Doppler without affecting gain. Still a further object of the present invention to significantly reduce the output data rate of the radiometer.
According to the present invention, digital Fast Fourier Transform (FFT) techniques are applied to passive microwave sounding in the measurement of an oxygen absorption line. The present invention provides a set of noise temperature measurements that are frequency contiguous and provide a temperature profile having high vertical resolution.
Other objects and advantages of the present invention will become apparent upon reading the following detailed description and appended claims, and upon reference to the accompanying drawings.